[#173] Migrate cql-antlr from its own repo to a submodule

[tolbertam]

From scassandra/cql-antlr#3 we discussed moving cql-antlr in as a submodule to make it easier to coordinate changes in scassandra that require changes to cql-antlr.

This works good, but currently there is something I need to fix in the gradle-build for cql-antlr. It builds properly, but the travis build fails because successive runs don't properly skip the antlr4 task that was previously run because i currently have it set up to generate license headers in the generated files. This is so the license checker doesn't fail. I could disable generating license headers for generated code, but then the license checker fails because excludes option in the checker doesn't work for generated code (since it evaluates which code to exclude first). Anyways..I plan on finding a strategy for that tomorrow. This is only a problem for the travis build because it builds with java 8, then runs tests with java 6.

This does not include the changes for scassandra/cql-antlr#3, I will incorporate those in #171 after this pr is merged.

I also was able to import the commit history from cql-antlr repo into this one. I didn't even know that was possible, neat!

[tolbertam]

The shared history thing doesn't appear to do much good since it still attributes the entire history of the file to me, sure the commits are there, but there isn't anything too substantial in the history to justify bringing it over I think, so I may get rid of it to keep the commit history clean.

[tolbertam]

Hard to believe 24 days has passed since I last looked at this. Working on finishing this up today. I cleaned up the history. Instead of migrating the commit history from the original repository to this one, I simply copied the repository in as is and attributed @chbatey as the committer. Then followed on with the extra two commits to fully integrate into this repository.

[tolbertam]

Alright, looks like I got it all sorted out (finally figured out how to exclude generated files from the license checker). This should be good to go!

[chbatey]

took me a while to clock you'd preserved the history!

[tolbertam]

Thanks for reviewing and merging :) 👍